Backlight systems for liquid crystal displays

ABSTRACT

A backlight system for a liquid crystal display includes a substantially planar, refractive waveguide having a first major face and a second major face opposite the first major face. The waveguide includes a viewable region corresponding to a viewable area of the liquid crystal display. The system further includes a light source positioned proximate to the second major face and within the viewing region for producing light. An injection feature is proximate to one or more of the second major face and the first major face and within the viewing region to optically couple the light into the waveguide such that the light becomes waveguided light. A plurality of extraction features is proximate to one or more of the second major face and the first major face and within the viewing region to optically couple the waveguided light out of the waveguide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 11/942,398, filedNov. 19, 2007.

TECHNICAL FIELD

The present invention generally relates to the field of liquid crystaldisplays (LCDs), and more particularly to direct backlight systems ofLCDs.

BACKGROUND

Liquid crystal display (LCD) monitors are replacing traditional cathoderay tube (CRT) monitors in many applications because of their lighterweight and superior performance. In a typical LCD, a backlight system isplaced behind an LCD panel to illuminate the LCD for viewing by a user.An array of light emitting diodes (LEDs) is used as the light source ofthe backlight system, although other sources of illumination can beprovided.

Conventional backlight systems typically fall into one of the followingtwo categories: direct backlight systems or edge backlight systems. Adirect backlight system typically has a light source directly behind theLCD panel with an integrating cavity therebetween that enables mixing ofthe light from the light source, thereby improving the uniformity of thedisplay. Conventional direct backlights can be problematic, however, inthat the cavity can result in an undesirable added thickness. Edgebacklight systems include light sources located at the edge of awaveguide (or “light pipe” or “light guide”) placed behind the LCDpanel. The light travels from the edge of the light guide until it isdeflected towards the LCD panel. Although conventional edge backlightsystems may be thinner than conventional direct backlight systems, suchdisplays often fail to provide sufficient luminescence (or “brightness”)for certain applications because the number of light sources is greatlyreduced and because the light must propagate throughout the entire lightguide from the edge of the display.

Accordingly, it is desirable to provide an improved backlight system forLCDs. In addition, it is desirable to provide a more compact backlightsystem with uniform luminescence. Furthermore, other desirable featuresand characteristics of the present invention will become apparent fromthe subsequent detailed description of the invention and the appendedclaims, taken in conjunction with the accompanying drawings and thisbackground of the invention.

BRIEF SUMMARY

In accordance with an exemplary embodiment, a backlight system for aliquid crystal display includes a substantially planar, refractivewaveguide having a first major face and a second major face opposite thefirst major face. The waveguide includes a viewable region correspondingto a viewable area of the liquid crystal display. The system furtherincludes a light source positioned proximate to the second major faceand within the viewing region for producing light. An injection featureis proximate to one or more of the second major face and the first majorface and within the viewing region to optically couple the light intothe waveguide such that the light becomes waveguided light. A pluralityof extraction features is proximate to one or more of the second majorface and the first major face and within the viewing region to opticallycouple the waveguided light out of the waveguide.

In accordance with another exemplary embodiment, a backlight system fora liquid crystal display includes a substantially planar waveguideincluding a first major face and a second major face opposite the firstmajor face. The waveguide includes a viewable region corresponding to aviewable area of the liquid crystal display. A light source ispositioned proximate to the second major face and within the viewingregion for producing light, and an injection feature is positionedproximate to at least one of the second major face and the first majorface and within the viewing region to optically couple the light intothe waveguide such that the light becomes waveguided light. A pluralityof extraction features is proximate to at least one of the second majorface and the first major face and within the viewing region to opticallycouple the waveguided light out of the waveguide. The plurality ofextraction features has an extraction density that varies.

In accordance with yet another exemplary embodiment, a liquid crystaldisplay (LCD) includes an LCD panel having a plurality of pixels and abacklight system coupled to and illuminating the pixels to form animage. The backlight system includes a substantially planar dielectricwaveguide including a first major face and a second major face oppositethe first major face. The waveguide includes a viewable regioncorresponding to a viewable area of the liquid crystal display. A lightsource is positioned proximate to the second major face and within theviewing region for producing light. An injection feature is within theviewing region to optically couple the light into the waveguide viarefraction such that the light becomes waveguided light. A plurality ofextraction features with an extraction density that varies is within theviewing region to optically couple the waveguided light out of thewaveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and wherein:

FIG. 1 is a cross-sectional view of an exemplary liquid crystal display(LCD);

FIGS. 2-22 are views of several exemplary injection features and lightsources;

FIGS. 23 and 24 are cross-sectional views of exemplary backlightsystems;

FIG. 25 is a graph illustrating the spread function of the backlightsystems of FIGS. 23 and 24;

FIGS. 26 and 27 are planar views of exemplary backlight systems;

FIGS. 28-31 are cross-sectional views of several exemplary extractionfeatures; and

FIGS. 32-36 are views of backlight systems with light sources havingdiffering spectral or color characteristics.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by any theorypresented in the preceding background or the following detaileddescription.

Exemplary embodiments discussed below provide liquid crystal displays(LCDs) having waveguides with injection features that refract a majorityof light from light sources into the waveguide such that the light iscontained within the waveguide via total internal reflection (TIR) untilextracted by extraction features. Other embodiments include waveguideshaving extraction features with varying extraction densities. Thedisclosed embodiments provide a compact backlight system with enhancedlateral spreading, mixing, and luminance.

FIG. 1 is a cross sectional view of an exemplary LCD 100. The LCD 100includes a direct backlight system 104 coupled to an LCD panel 102.During operation, the backlight system 104 provides output light 132 toenable a viewer to view pixel patterns on the LCD panel 102 that form animage. The LCD 100 can be used in any display application, includingavionic displays.

In a typical LCD panel 102, there is an active matrix array of manythousands of pixel structures. Although not described in greater detailfor brevity, the LCD panel 102 may include, in one exemplary embodiment,any addressing structure such as a structure that includes thin filmtransistors processed onto a lower glass plate, cells of liquid crystalmaterial, a common electrode adjacent to the liquid crystal material,color filters processed onto an upper glass plate, and a pair ofappropriately oriented linear polarizing films. If desired, an optionaltransmissive diffuser 103 may be included with LCD panel 102 to furtherblend and homogenize output light 132. An air gap may be includedbetween the diffuser 103 and the LCD panel 102. Any light reflected bydiffuser 103 will be returned to the backlight for another chance to bescattered, deflected or reflected by the various backlight componentsbefore rejoining output light 132. Other enhancement films, such asprismatic or lenticular films, reflective or scattering polarizer films,and various types of diffusion films, may also be provided on oradjacent to the LCD panel 102 in the path of output light 132.

Generally, the backlight system 104 includes a viewable region 105extending in front and behind the backlight system 104 that correspondsto an area of the LCD 100 viewed by a viewer. The backlight system 104includes a unitary, refractive waveguide 106 formed from a transparentoptical material such as glass, acrylic, polycarbonate, transparentpolymers, or similar materials. Waveguide 106 has one or more edges 112and two major faces 108 and 110 that are substantially parallel to eachother. The backlight system 104 further includes one or more lightsources 116, such as light emitting diodes (LEDs) or other lightsources, distributed across the viewing region 105. The light sources116 are optically coupled to the waveguide 106 by injection features 114distributed across the viewing region 105 such that light (e.g., ray124) from the light sources 116 enters the waveguide 106 via refraction.

As described in further detail below, the light is effectively confinedwithin the waveguide 106 until reaching an extraction feature 128 thatdirects light out of the waveguide 106 for illumination of the LCD panel102. In alternate embodiments, some of the light sources 116, injectionfeatures 114 and/or extraction features 128 are located out of theviewing region 105. The backlight system 104 can further include areflective layer 136 behind the waveguide 106 and behind or adjacent thelight sources 116. Reflective layer 136 serves to redirect rays whichhappen to be extracted in the opposite direction from output light 132,or are otherwise aimed away from LCD panel 102.

The waveguide 106 is characterized as being a unitary refractivestructure in that substantially all of its distributed substructures,specifically its major faces 108, 110, injection features 114 andextraction features 128, are refractive in nature, comprising refractivematerials and interfaces, for example clear plastic and air. Light raysincident on refractive interfaces follow well-characterized propertiesof transmission, reflection or total internal reflection (TIR),depending upon the refractive indices and angles of incidence. Certainother embodiments, described below, may not meet the strict requirementsof a “unitary waveguide” in that they may include reflective mirrors,pigments, volume diffusers or other substructures not easilycharacterized by the laws of refraction. Some will still however containunitary injection features or unitary extraction features, dependingupon the detailed design and constituent structures.

In the depicted embodiment of FIG. 1, the injection features 114 areconical and appropriately designed, formed, and polished to effectivelyhide the associated light source 116 from direct visibility by injectingsubstantially all of the light. Additional examples of injectionfeatures will be described below, each of which can offer potentialadvantages such as ease of fabrication, support of different lightsource topologies, or use of more efficient or environmentally suitableoptical materials.

In the depicted embodiment of FIG. 1, the extraction features 128 areconical, and extract most or all of the waveguided light from thewaveguide 106. The extraction features 128 can be varied as a functionof number, size, geometry, and position from the injection features 114,and position relative to each other. These parameters result in a set ofextraction features 128 with a given extraction density, whichrepresents the amount or fraction of light extracted from the waveguideover a given area. Interference with the TIR of the waveguided light isone exemplary extraction mechanism. This can be accomplished by eitherdeflecting wave-guided light rays or by localized deviations in thewaveguide surface. The extraction features 128 depicted in FIG. 1 anddiscussed below can utilize one or both of these mechanisms to causelight to be extracted from waveguide 106.

In one embodiment, injection features 114 can also function asextraction features when waveguided light strikes the injection feature114 and is directed out of the waveguide 106. Similarly, light incidenton extraction features 128 can be effectively injected into waveguide106. For example, light extracted by extraction feature 128 may strikereflective layer 136 and be injected back into waveguide 106 by one ormore of the extraction features 128. Any single ray can interact with asingle feature or a number of features and interfaces before finallyexiting as output light 132.

The exemplary embodiment depicted in FIG. 1 may have a thickness, ordepth, that is relatively thin compared with some prior art directbacklights having comparable separation between adjacent light sources116. In some embodiments, a small cavity or separation between waveguide106 and LCD panel 102 is provided to enhance mixing, especially inembodiments that include a diffuser 103, described above. In otherembodiments, no cavity is necessary since the light is adequately mixedin the waveguide 106. The distance 142 between the reflective layer 136and the LCD panel 102 represents the optical depth of the backlight,including any included diffusers and air gaps, and can be less than thelateral separation 140 between adjacent light sources 116, particularlynearest neighboring light sources 116 having similar colorcharacteristics, more preferably less than half the lateral separation140, and even more preferably less than 25 percent of the lateralseparation 140. This embodiment is generally more readily scalable thanedge lit designs, and allows distribution of the heat generated by lightsources 116 over a larger area. In another embodiment, the inventiontakes the form of a waveguide 106 for insertion into a conventionaldirect backlight having an appreciable distance 142, for example adistance 142 of 0.75 inches or greater, but in this case a hightransmission diffuser 103 can be used in place of a conventional directbacklight diffuser. In yet another embodiment, distance 142 ispreferably three or more times the thickness of waveguide 106, with theextra integrating volume containing an air cavity between waveguide 106and high transmission diffuser 103. Generally, the waveguide 106 is hasa locally average thickness that is substantially constant across theviewable region.

The operation of an exemplary injection feature 200 of a waveguide 202is more clearly shown in the cross-section view of FIG. 2. The waveguide202 is a transparent optical material, and in this example, is anacrylic. Injection feature 200 is an indentation, generally conical inshape, and filled with a lower index medium such as air. The injectionfeature 200 couples the waveguide 202 to a light source 206. Thebehavior of the light rays generated by light source 206 is based on therefractive indices of the injection feature 200 and the waveguide 202and the geometry of the injection feature 200. As such, these parameterscan be manipulated to enhance the waveguided light within the waveguide202. Particularly, the parameters can be manipulated to ensure that asmuch light as possible, preferably a majority of the light, and morepreferably substantially all of the light from the light source, isinjected and meets the conditions for TIR within the waveguide 202.

As one example, ray 211 from the light source 206 strikes surface 220 ofthe injection feature 200. A resulting, refracted and waveguided ray 212can be predicted based on the angle 224 of the injection feature 200 andthe respective refractive indices, which in this case is 1.0 for air andn≈1.49 for acrylic. In order to consider the ray waveguided, andtherefore injected into the waveguide 202, ray 212 must exceed a certainangle at major face 203 to be reflected via TIR. TIR occurs when theangle 222 between ray 212 and the normal to major face 203 of waveguide202 exceeds a sin(sin(90°)/n), or about 42° in this case. By setting thecone angle 224 of the injection feature such that the refracted ray 212makes an equivalent angle with the injection surface 220, then any lightfrom light source 206 entering the injection feature will be injected.This yields a cone angle 224 of approximately 2*(90-2*42)=12° foracrylic. The equivalent angle 224 for polycarbonate waveguide (n=1.59)would be around 24 degrees, and the cone of the injection feature 200could be even less steep if higher refractive index materials such ashigh index glass were used. Larger cone angles 224 of the injectionfeature 200, corresponding to less sharply pointed cones, can also beused if the size of the light sources 206 is smaller than the base 208of the injection feature 200, or if complete injection of the light isnot required. Alternate transmissive materials within the injectionfeatures 200, such as clear silicone or other adhesives or polymers,with other refractive indices may be used instead of air for betterindex-matching, with corresponding changes in the refracted rays. Uponreflection, ray 212 continues to propagate through the waveguide 202until the ray 212 strikes an extraction feature, as discussed in furtherdetail below. In the present embodiment, waveguide 202 is a singleextended piece, but in other embodiments waveguide 202 includes multiplesmaller waveguides between which at least a portion of the waveguidedrays can pass.

FIGS. 3-22 depict several exemplary injection features and light sourcesthat can be used in the backlight systems described herein to inject,either primarily or completely via refraction, a majority or morepreferably substantially all of the light from the light source suchthat the light remains confined within the waveguide due to TIR untilthe light is extracted by an extraction feature. The injection featurescan be cast, molded, or otherwise formed in or adjacent the waveguide.

As one example, FIG. 3 is a cross-sectional view of an exemplaryinjection feature 400 coupling light source 402 to a waveguide 404. Theinjection feature 400 includes a first cone 406 on a first major face408 of the waveguide 404 and a second cone 410 on a second major face418 of the waveguide 404. The opposing first and second cones 406 and410 enable a broader cone angle as compared to, for example, theembodiment depicted in FIG. 2. The broader cone angles 412 and 414 mayenable a relatively thinner waveguide 404. The light source 402 is anon-flat LED at least partially extending into the first cone 406,although in other embodiments, the light source 402 is a flat,side-emitting, Lambertian, or directional LED, or other source having adifferent source geometry or angular output profile.

FIG. 4 is a cross-sectional view of another exemplary injection feature420 coupling light source 422 to a waveguide 424. In this example, theinjection feature 420 is tapered or conical with a truncated end 426,which enables a relatively thin waveguide 424. To minimize leakage ofnon-injected light directly out of the waveguide 424, an insert 428,either a specular or diffuse reflector, may optionally be providedwithin the injection feature 420 at the truncated end 426. In otherembodiments, the insert 428 can be white, partially transmissive,adhesive, paint, fill material, an LED cap, and/or a tinted underside.

FIG. 5 is a cross-sectional view of another exemplary injection feature440 coupling light source 442 to a waveguide 444. In this example, theinjection feature 440 is cylindrical with an optional reflector ormasking element 448 at one end 446. The light source 442 is a sideemitting LED, which may minimize the amount of light that would reachend 446 or element 448 prior to injection through the side wall 450.

FIG. 6 is a cross-sectional view of yet another exemplary injectionfeature 460 coupling light source 462 to a waveguide 464. In thisexample, the injection feature 460 is a cylindrical through hole, whichis relatively simple to fabricate in that no angular walls within thewaveguide 464 are necessary. A mask 466, either reflective, scattering,absorbing or a combination thereof is optionally positioned at an end ofthe injection feature 460 opposite the light source 462. In variousembodiments, the mask 466 can be a white sheet with cutouts, paint,screen printing, tape, adhesive, patterned sheets, and/or partiallytransmissive or tinted materials.

FIG. 7 is a cross-sectional view of another exemplary injection feature480 coupling light source 482 to a waveguide 484. In this example, theinjection feature 480 is a curved conical shape on a major face 486 ofthe waveguide 484 opposite the light source 482. In various embodiments,the injection feature 480 can be curved, multi-faceted or otherwisecomplex. Similarly, the simpler conical or cylindrical structures ofother embodiments can alternately be curved or multi-faceted as well.While it is preferred for the embodiment of FIG. 7 that light source 482has a somewhat directional output, this is not required. The lightsource 482 may also include internal side reflectors or other opticalmechanisms to assist the directional output.

FIG. 8 is a cross-sectional view of an exemplary embodiment in whichlight is directed directly into a waveguide 500 by a light source 502.In this example, a backscattering layer 504, such as white pigment orpaint, is provided on a major face 506 opposite the light source 502 toscatter and reflect the light such that a substantial portion of it isinjected and waveguided in a lateral direction. Light source 502 ispreferably a directional light source, although this is not required.

FIG. 9 is a cross-sectional view of another exemplary embodiment inwhich light is directed into a waveguide 510 by a light source 512. Inthis example, an immersed oblique reflective structure 514 serves as theinjection feature, injecting the light such that it is waveguided in alateral direction.

FIG. 10 is a cross-sectional view of another exemplary embodiment andincludes an injection feature 520 that injects light from two lightsources 522, 524 into a waveguide 526. The injection feature 520 in thisexample is a truncated cone, and a masking layer 528 is provided toassist the injection feature 520 in injecting the light into thewaveguide 526. The masking layer 528 can be applied, for example, byscreen printing a diffuse white or specular layer over a major face 530of the waveguide 526 opposite the light sources 522, 524.

FIG. 11 is a cross-sectional view of another exemplary embodiment andincludes an injection feature 540 that injects light from a light source542 into a waveguide 544. In this example, the injection feature 540 iscylindrical and the light source 542 is a side-emitting LED. The maskinglayer 546 can be provided on a major face 548 of the waveguide 544opposite the light source 542.

FIG. 12 is a cross-sectional view of another exemplary embodiment andincludes an injection feature 560 that injects light from a light source562 into a waveguide 564. In this example, the injection feature 560 iscylindrical. A plug 566 is provided in the injection feature 560opposite the light source 562 to block at least a portion of the light.

FIG. 13 is a cross-sectional view of another exemplary embodiment andincludes an injection feature 580 that injects light from a light source582 into a waveguide 584. In this example, the injection feature 580 iscylindrical. A plug 586 can be provided in the injection feature 580opposite the light source 582 to block at least a portion of the light.In contrast to the plug 566 in FIG. 12, the plug 586 has beveled orotherwise angled surfaces that may improve injection into the waveguide.

FIG. 14 is a cross-sectional view of another exemplary backlight system620 that includes a waveguide 622 coupled to a light source 630 byopposing portions 632, 634 of an injection feature 636. The waveguide622 includes a first substrate 624 and a second substrate 626. The firstsubstrate 624 and the second substrate 626 are optically bonded togetherin a manner such that the substrates 624 and 626 are substantiallyindex-matched, meaning that there is not a low index gap such as an airgap between them. This results in at least a majority of any waveguidedlight being passed back and forth freely between substrates 624 and 626.Such bonding can be achieved with optical adhesives or by a variety ofother methods, such as thermal, mechanical or chemical processes. Othermultiple substrate embodiments comprise one or more other injectionfeature implementations described above.

FIG. 15 is a cross-sectional view of another exemplary backlight system640. The backlight system 640 includes a waveguide 642 coupled to alight source 650. The waveguide 642 includes a first substrate 644 and asecond substrate 646. The first substrate 644 may capture at least aportion of any rays from the light source 650 that pass through thesecond substrate 646 without being injected. The waveguide 642 alsoincludes a gap 648 between the first and second substrates 644, 646,although localized optical bonding or contact can occur in selectedlocations, using structures similar to those described in reference toFIG. 14. Extraction features, such as those described in reference FIG.1 or below, can be arranged in one or both of the substrates 644, 646.

FIG. 16 is a cross-sectional view of another exemplary backlight system660. The backlight system includes a waveguide 662 coupled to a lightsource 670. An attenuating mask layer 672 overlays the waveguide 662 toblock or attenuate a direct path from the light source 670 out of thewaveguide 662.

FIG. 17 is a cross-sectional view of yet another exemplary backlightsystem 680. The backlight system 680 includes a waveguide 682respectively coupled to light sources 684, 686 with injection features688, 690. Although still substantially parallel, the waveguide 682includes wedged portions 692, 694 that open up space 696 in between. Asa result, the waveguide 682 can have a relatively smaller averagethickness and weight and/or the ability to accommodate additionalfeatures within the space 696.

FIG. 18 is a cross-sectional view of another exemplary backlight system940 that includes light sources 941-943 coupled to a waveguide 944. Inthis embodiment, light is injected by a combination of individualinjection features 945-947 and a shared injection feature 948. Theinjection feature 948 has a top portion 950 and a tapered bottom portion952, which is better shown in the top plan view of FIG. 19.

The majority of the injection embodiments thus far have been describedin the context of being symmetrical around a vertical axis of symmetry,such as conical or cylindrical. In other embodiments, related structuresmay have alternate symmetries, for example pyramidal or rectangular, oreven be fundamentally asymmetric, for example with an upper portionbeing slightly offset with respect to the bottom portion. Yet anotherexemplary symmetry variant is shown in FIG. 20. FIG. 20 is an isometricview of a waveguide 800 suitable for use in the backlight systemsdescribed herein. The waveguide 800 includes a plurality of injectionfeatures 802 and extraction features 804. In this embodiment, theinjection features 802 can accommodate linear light structures such as afluorescent lamp or rows of LEDs distributed across the viewable region.The use of linear injection features 802 or extraction features 804 mayresult in the waveguide 800 having an asymmetric light emitting pattern.In alternate embodiments, injection features 802 can accommodate acombination of linear and point light sources such as the LEDs describedabove. In other embodiments, each of the injection features disclosedabove can be extended in a manner such as this, or the various types andorientations of all of the injection features and extraction featuresmay be mixed and matched within or on the waveguide.

FIG. 21 is a cross-sectional view of another exemplary backlight system600. The backlight system 600 includes a waveguide 602 coupled to alight source 610. The waveguide 602 includes flat portions 604 andobliquely angled portions 606. A conical injection feature 608 in theangled portion 606 couples the waveguide 602 to a light source 610. Thewaveguide 602 has a generally constant thickness throughout the viewableregion and may be thinner than other embodiments, allowing for reducedwaveguide weight. The generally constant average thickness improvescompatibility with certain manufacturing processes such as compressionmolding, since removal of material is unnecessary in forming the unitaryrefractive structure. Only local material flow is required to form thedetailed optical surfaces. The embodiment is shown with a flat emitter,but as is the case with the other embodiments, nearly any emittertopology can be utilized. Moreover, any suitable extraction features(not shown) can be used. In another embodiment, the injection feature608 has the cross-section shown in a first axis, and extends linearly ina second axis, such as was shown for injection feature 802 in FIG. 20.In yet another embodiment, waveguide 602 is formed in discrete sections,for example the right and left sides in FIG. 21, which are abutted orjoined above light source 610.

The geometry of the flat and angled portions 604, 606 of the waveguide602 accommodates additional circuitry 611 between the light source 610and adjacent light sources (not shown). In some embodiments, allinterface and drive circuitry for an LCD system resides on the sameplane or board as the one or more light sources 610. The backlightsystem 600 further includes a distributed heat sink 612 for effectivelyspreading and removing the heat. The heat sink 612 is correspondinglyscalable with the circuitry 611 and light source 610.

FIG. 22 illustrates a portion of a backlight system that can be used inconjunction with the injection features described above. Particularly,FIG. 21 is a block diagram of exemplary light source circuitry 700 usedto drive the light sources of the backlight systems described herein.Light sources 762 are grouped into three groups 763, 764, 765. Eachlight source 762 includes a driver 772 coupled to an LED 774. Eachdriver 772 couples to a common signal and power bus connection 770allowing complex driving and distribution of the supplied current. Thelight sources 762 can be dynamically driven as individual LEDs, asgroups 763-765, or collectively as an entire system. The drive circuit772 is optionally contained on a circuit board with the LEDs 758 andresides within the lateral gaps between adjacent LEDs 774. Light spreadsuniformly between the groups 763-765, which can represent a regulararray of sources, distinctly separate source modules injecting lightinto a larger waveguide, or any other suitable physical layout. Thisembodiment also enables the suppression of hot spots at the LEDs 774 aswell as at distinct source modules. In another embodiment, the LEDs 774in groups 763, 764, 765 are driven as one or more series strings ofLEDs.

In a variation of the embodiment of FIG. 22, some of the LEDs 774 ofFIG. 21 can be replaced with diodes which are non-emissive, or withpassive resistive loads. By selectively driving the non-emissive loadsto generate heat, the temperature of the backlight can be raised ormaintained independently of the brightness setting. This can be usefulfor maintaining consistent display performance or even for warming adisplay panel under cold environment conditions, and the effectivenessof the technique is enhanced by the reduced distance and distributedarrangement of the emissive and non-emissive sources of heat. In thisvariation, either a complex or simplified drive scheme can be utilized.The non-emissive loads can be driven by a separate power source capableof being modulated to adjust the desired rate of heat generation. Ifdesired, thermal conductivity can be included as one of the relevantparameters considered during the process of selecting materials for acorresponding waveguide and other backlight components in order tominimize temperature differences between the display panel and thebacklight system.

While FIGS. 4-22 illustrate various types of light sources and injectionfeatures, FIGS. 23-27 illustrate several techniques for extractingwaveguided light out of the waveguide. For example, FIG. 23 is across-sectional view of a backlight system 820 having a waveguide 822coupled to a light source 824 by injection feature 826, and FIG. 24 is across-sectional view of another backlight system 840 also having awaveguide 842 coupled to a light source 844 by injection feature 846.The backlight system 820 of FIG. 23 has relatively small extractionfeatures 828 as compared to the extraction features 848 of the backlightsystem 840 of FIG. 24. The relative sizes of the extraction features828, 848 result in differences between the spatial extent of the spreadfunction of the light in a lateral direction, which is illustrated bythe graph of FIG. 25, as the larger extraction features 848 extract morelight than the smaller but comparably spaced extraction features 828. Interms of extraction density, each of backlight system 820 and 840 has anextraction density which varies spatially, but backlight system 840 hasa generally higher extraction density than backlight system 820. Inthese embodiments, this is because the extraction features are largerand more effective while the spacing of the extraction features iscomparable. Line 850 in FIG. 25 represents the amount of lightwaveguided and extracted from the waveguide 822 as a function of thedistance from the light source 824, and line 852 in FIG. 25 representsthe amount of light waveguided and extracted from the waveguide 842 as afunction of the distance from the light source 844. As such, backlightsystem 820 has a more intense amount of light at and immediatelysurrounding the light source 824, but the backlight system 840 moreevenly distributes light from light source 844 over a greater area. Theeffective spread function is therefore determined in large part by theextraction density of the detailed extraction feature design andspacing, and can impact redundancy, color mixing effectiveness, andfurther topics such as dynamic backlight techniques. The wider spreadfunction represented by line 850 is an indication that extractionfeatures 828 have a generally lower extraction density than extractionfeatures 848 which result in the narrower spread function represented byline 852.

FIG. 26 is a planar view of an exemplary backlight system 300 andillustrates the manipulation of the extraction density and the relatedspread function. In this view, injection features 302 are represented byrelatively large circles. Extraction features 325 are represented bysmaller circles. The injection features are arranged in a regular squarearray with equal separation in both horizontal and vertical directions,e.g., horizontal distance 320 and vertical distance 321, but thisarrangement is not necessary. The pattern of injection features 302 andextraction features 325 could be asymmetric rectangular, hexagonal,random, or any other two dimensional array.

The injection features 302 and extraction features 325 are arranged intoregions 330. The regions 330 are further divided into one or moresubregions 340. While distinct subregions are depicted for clarity ofexplanation, it should be understood that continuously varyingdistributions of extraction density is a more general case. In thedepicted embodiment, the backlight system 300 includes sixteen regions330, each with one injection feature 302, and each region 330 includestwenty-five subregions 340, one of which coincides with injectionfeature 302. The subregions 340 are defined by one or more extractionfeatures 325, in this case, nine extraction features 320. Typically, theextraction features 325 of a particular subregion 340 have a particularextraction density. As noted above, extraction density corresponds tothe degree by which light is extracted by a particular area ofextraction features. The extraction density can be varied, for example,by adjusting the feature density, feature size, feature shape or type ofextraction features, and as described previously, facilitates thecapability of making the output uniform for a wide variety of LEDconfigurations and waveguide materials. Each of the regions 330 andsubregions 340 can have varying extraction densities. In thisembodiment, the extraction densities of the subregions 340 aremanipulated such that light from a respective injection feature 302spreads evenly throughout the region 330, but without significant spreadinto adjacent regions 330. In other words, the extraction featuretopology provides a relatively symmetric and localized spread function.

In one embodiment, the injection features 302 can contribute to aninjection leakage density, which is a measure of how much lighttransmitted directly through the waveguide without being injected.Certain embodiments described herein attempt to minimize the injectionleakage density. However, in other embodiments, the extraction densitycan be tuned to the injection leakage density such that the output lightis uniform.

FIG. 27 is a plan view of an exemplary backlight system 860. Thebacklight system 860 has a plurality of injection features 862 andextraction features 864 distributed through a waveguide 866. Theextraction features 864 vary in size throughout the waveguide, asindicated by the relative size of the dots representing the extractionfeatures 864. In this embodiment, the topology of the extractionfeatures 864 has an asymmetrical design as compared to the moresymmetrical design of FIG. 26. The extraction features 864 in anx-direction are relatively constant while the extraction features 864 ina y-direction are more varied. As a result, the light injected frominjection features 862 extends to a greater extent (i.e., a broaderspread function) in the x-direction than in the y-direction. This can beparticularly useful in applications in which mixing is desired along thex-direction, but not the y-direction, such as dynamic backlightingtechniques that synchronize the backlight with the row or column updatetiming progression or to conserve power or enhance visual contrast byspatially modulating the backlight system 860. In an embodiment of thepresent invention, an asymmetrical spread function embodiment iscombined with the independently dynamic drive embodiment as described inFIG. 22, facilitating dynamically addressable rows of illumination in areduced depth configuration. In another embodiment, the symmetrical butnarrower spread function embodiment of FIG. 26 are combined with theembodiment of FIG. 22, facilitating dynamically addressable regions of acompact backlight. In yet another embodiment, a broad spread function isutilized in both x and y directions to facilitate enhanced mixing oflight from a plurality of LEDs.

FIGS. 28-31 illustrate various types of extraction features that can beincorporated in the backlight systems discussed herein. The extractionfeatures can be cast, molded, or otherwise formed. FIG. 28 iscross-sectional view of a portion of a backlight system 720 having awaveguide 722 with top and bottom major faces 724, 726 and a pluralityof extraction features 728-735. Extraction features 728, 729 are wedgeshaped and are formed internal to the bottom major face 726 and topmajor face 724, respectively. Extraction features 730, 731 are irregularand formed on the bottom major face 726 and top major face 724,respectively. Extraction features 732, 733 are dimple shaped and formedinternally on the bottom major face 726 and the top major face 724,respectively. Extraction features 734, 735 are a series of regular wedgeshaped or prismatic groove features formed in the bottom major face 726and the top major face 724. While only a first cross-section is shown,it should be understood that the other cross-sections can besymmetrical, can be different, or can comprise linear structures asdescribed above. Making the physical cross-sections different, forexample slightly broadened in one axis, or the fully extended example ofFIG. 20, allows additional flexibility in achieving uniform output,since the extraction features then have varying angular cross-sections.This can be leveraged to provide extraction densities that depend uponorientation relative to the ray propagation direction as well as spatiallocation.

As further examples of extraction features, FIG. 29 is a cross-sectionalview of a portion of a backlight system 740 having a waveguide 742 withtop and bottom major faces 744, 746 and a plurality of extractionfeatures 748-756. Extraction features 748, 749 are wedge shaped andextend from the bottom major surface 746 and the top major surface 744.Extraction feature 750 is an internal, irregular inclusion in thewaveguide 742. Extraction feature 751 is dimple shaped and extends fromthe top major surface 744. Extraction feature 752 comprises a localizedregion in which a diffusely reflecting layer 757, such as reflectivelayer 136 in FIG. 1, is locally index-matched by optical bond 710 tomajor surface 746 of waveguide 742. Optical bond 710 can be, forexample, a clear adhesive layer such as an applied and cured polymer ora patterned transfer adhesive layer. Extraction feature 753 includes anoptically structured upper layer 758 which is locally index-matched totop major surface 744 by optical bond 712. Extraction features 754, 755are externally applied diffusing layers on the bottom major surface 746and the top major surface 744. A preferred material for extractionfeature 754 or 755 is highly reflective and scattering white paint or arelated structure. Extraction feature 756 is another example having adiffuser layer 759 with a diffuse surface texture and locallyindex-matched to top major surface 744 by optical bond 714.

FIG. 30 is a cross-sectional view of a portion of a backlight system 760with more examples of various types of extraction features 764-768.Extraction feature 764 is conical shaped and unpolished. Extractionfeature 766 is more cylindrical than extraction feature 764 and is alsounpolished. Extraction feature 767 has a profile well-suited to extractlight toward a scattering reflector such as reflector 136 of FIG. 1. Bykeeping the depth of the extraction feature 767 small compared with itswidth, only a small fraction of the extracted and diffusely reflectedlight will be re-injected through the side walls. Extraction feature 768is an example of a stepped wedge light extraction structure.

FIG. 31 is a cross-sectional view of a portion of yet another backlightsystem 780. In this embodiment, the waveguide 782 includes a top portion784 and a bottom portion 786. The top portion 784 and bottom portion 786are bonded together by an index matching adhesive or other mechanism.The bottom portion 786 can be considered, for example, a secondary layeror film. Injection features 788 are arranged throughout the waveguide782 to couple the waveguide 782 to a light source (not shown), andextraction features 790 are arranged to direct light out of thewaveguide 782. The injection features 788 are formed in both the topportion 784 and bottom portion 786 of the waveguide while the extractionfeatures 790 are formed on the bottom portion 786. Light extracted fromwaveguide 782 by extraction features 790 may be redirected by a rearreflector, such as reflective layer 136 of FIG. 1.

FIGS. 32-37 illustrate exemplary backlight systems with light sourceshaving different spectral or color characteristics. For example, fullcolor may be achieved via white light sources (W), a mixing of colorlight sources such as red, green and blue (RGB), or by mixing both whiteand colored light sources in varying combinations, such as RW, RBW, RGBWand so forth. Embodiments disclosed herein allow effective mixing of anynumber or combination of light source contributions while maintaining alow profile or depth and without a substantial extension of componentsbeyond the viewable region. In addition, the number of light sources foreach color component can be different, allowing considerable flexibilityin adjusting the color gamut, chromaticities or detailed spectralproperties of the backlight and resulting display system.

FIG. 32 is a cross-sectional view of an exemplary backlight system 880that includes a waveguide 881, and a plurality of injection features882-885 and extraction features (e.g., 886-891) that respectively injectand then extract light generated by light sources 892-895. Light source893 is a red light source, and light source 894 is a blue light source.Light sources 892, 895 are white light sources. In this example, red andblue light from light sources 893, 894 is injected through therelatively smaller injection features, e.g., injection features 883,884. The white light from light sources 892, 895 is injected through thelarger injection features, e.g., injection features 882, 885. Extractiondensity is varied across the extraction features, for example in theregion from extraction feature 890 to extraction feature 888, to yieldsubstantially uniform light output in conjunction with any leakage fromwhite light injection, for example light source 895 and injectionfeature 885. Injection feature 884 in this embodiment is acting as bothan injection feature and an extraction feature, allowing colored lightto be mixed in with other light in an effective manner.

FIG. 33 is a cross-sectional view of another exemplary backlight system900 that includes a waveguide 901, and a plurality of injection features902-907 and extraction features 908 that respectively inject and thenextract light generated by light sources 909-914. In this embodiment,the light sources 909-914 can be a combination of various colors, suchas red, green, blue and white. The extraction features 908 have a lowerprofile, and in particular a lower extraction density, than some otherembodiments to facilitate a broader spread function and an enhancedmixing of the light sources 909-914. The spread function in theorthogonal direction may either be comparably broad to facilitate mixingin that direction as well, or may be deliberately shorter as discussedabove, and as was seen in the embodiment of FIG. 27, if color mixing isless important in the second axis. In another embodiment, the multicolorsequence of light sources 909-914 can be situated under a linearinjection structure, for example injection feature 802 of waveguide 800in FIG. 20.

In one embodiment, such as shown in the plan view of FIG. 34, lightsources 915 can be grouped into clusters 916 distributed across anactive area region 917 of a display system 918. In a further embodiment,each cluster 916 contains one LED 915 of each color, for example R, G, Band W. In yet a further embodiment, each cluster 916 is injected into awaveguide such as waveguide 526 of FIG. 10 via injection feature 520 ofFIG. 10.

FIG. 35 is a cross-sectional view of another exemplary backlight system920 that includes a waveguide 921. Injection features 922 and extractionfeatures 923 respectively inject and then extract light generated bylight sources 924. The backlight system 920 further includes additionallight sources 925 that inject light through one or more edges 926 of thewaveguide 921.

FIG. 36 is a cross-sectional view of another exemplary backlight system930 that includes a waveguide 931. Injection features 932 and extractionfeatures 933 respectively inject and then extract light generated bylight sources 934. The backlight system 930 further includes lightabsorbing features 935 to tune a particular portion of spectrum andchromaticity of the backlight system 930 without introducing uniformityconcerns.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention. It being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

1. A backlight system for a liquid crystal display, comprising: asubstantially planar, refractive waveguide including a first major faceand a second major face opposite the first major face, the waveguideincluding a viewable region corresponding to a viewable area of theliquid crystal display; a plurality of light sources positionedproximate to the second major face and within the viewing region forproducing light; an injection feature proximate to one or more of thesecond major face and the first major face and within the viewing regionto optically couple the light into the waveguide such that the lightbecomes waveguided light; a plurality of extraction features proximateto one or more of the second major face and the first major face andwithin the viewing region to optically couple the waveguided light outof the waveguide; a plurality of drivers coupled to the plurality oflight sources and configured to dynamically drive the plurality of lightsources based on spatial location within the viewing area; and lightabsorbing features configured to tune backlight chromaticity byabsorbing a portion of visible spectrum from a first light source of theplurality of light sources.
 2. The backlight system of claim 1, whereinthe backlight system has an optical depth and further comprises areflective layer proximate to the second major face; and a transmissivediffuser proximate to the first major face, wherein the plurality oflight sources include a first light source and a second light source,the second light source having a similar color characteristic to thefirst light source, wherein the optical depth is less than a lateralseparation between the first light source and the second light source.3. The backlight system of claim 1, wherein the waveguide includes afirst region having a first localized spread function and a secondregion having a second localized spread function.
 4. The backlightsystem of claim 1, wherein the waveguide has a first spread function ina first direction and a second spread function in a second direction,the first and second spread functions being approximately the same, thefirst direction being generally orthogonal to the second direction. 5.The backlight system of claim 1, wherein the waveguide has a firstspread function in a first direction and a second spread function in asecond direction, the first and second spread functions being different,the first direction being generally orthogonal to the second direction.6. The backlight system of claim 1, wherein the plurality of extractionfeatures includes first extraction features with a first extractiondensity in a first direction and second extraction features with asecond extraction density in a second direction, the first directionbeing generally orthogonal to the second direction.
 7. The backlightsystem of claim 1, wherein the liquid crystal display is configured tobe driven according to timing signals, and wherein the plurality ofdrivers is configured to synchronize driving the plurality of lightsources with the timing signals.
 8. The backlight system of claim 1,wherein the liquid crystal display is defined by rows and columns, andwherein the plurality of drivers is configured to synchronize drivingthe plurality of light sources based on a timing progression of the rowsand columns.
 9. The backlight system of claim 1, wherein the pluralityof light sources includes light sources arranged in a plurality ofregions, and wherein the plurality of drivers is configured todynamically address individual regions of the plurality of regions. 10.The backlight system of claim 1, wherein the plurality of light sourcesincludes light sources arranged in a plurality of rows, and wherein theplurality of drivers is configured to dynamically address individualrows of the plurality of rows.
 11. The backlight system of claim 1,wherein the plurality of drivers is configured to spatially modulatesthe plurality of light sources.
 12. The backlight system of claim 1,wherein the plurality of light sources includes a plurality of lightemitting diodes (LEDs).
 13. The backlight system of claim 1, furthercomprising a transmissive diffuser proximate to the first major face;and a mixing cavity separating the first major face and the transmissivediffuser.
 14. The backlight system of claim 1, wherein the plurality oflight sources includes light sources arranged into source groupingsconfigured to provide light to the waveguide, the source groupings beingindividually driven and smaller than the waveguide.
 15. The backlightsystem of claim 1, wherein the waveguide is formed by discrete waveguidesections.
 16. The backlight system of claim 1, wherein the plurality oflight sources includes a first light source and a second light source,the second light source being adjacent to, and forming a gap with, thefirst light source, and wherein the backlight system further comprisescircuitry positioned within the gap between the first light source andthe second light source.
 17. The backlight system of claim 1, furthercomprising a non-emissive device proximate to the second major face andconfigured to generate heat.
 18. A backlight system for a liquid crystaldisplay, comprising: a substantially planar, refractive waveguideincluding a first major face and a second major face opposite the firstmajor face, the waveguide including a viewable region corresponding to aviewable area of the liquid crystal display; a plurality of lightsources positioned proximate to the second major face and within theviewing region for producing light; a plurality of injection featuresproximate to one or more of the second major face and the first majorface and within the viewing region to optically couple the light intothe waveguide such that the light becomes waveguided light; and aplurality of extraction features proximate to one or more of the secondmajor face and the first major face and within the viewing region tooptically couple the waveguided light out of the waveguide, wherein theplurality of extraction features includes extraction features arrangedin a first group and a second group adjacent to the first group, andwherein the waveguide includes a first area corresponding to the firstgroup of extraction features and a second area corresponding to thesecond group of extraction features, and wherein the first group ofextraction features defines a localized spread function such that lightin the first area spreads substantially evenly through the first areawithout substantially spreading into the second area.
 19. The backlightsystem of claim 18, further comprising light absorbing featuresconfigured to tune backlight chromaticity by absorbing a portion ofvisible spectrum from a first light source of the plurality of lightsources.